This invention relates generally to nuclear reactors and more particularly, to apparatus for repairing jet pump assemblies within a nuclear reactor pressure vessel.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
In a BWR, hollow tubular jet pumps positioned within the shroud annulus provide the required reactor core water flow. The upper portion of the jet pump, known as the inlet mixer, is laterally positioned and supported against two opposing rigid contacts within restrainer brackets by a gravity actuated wedge. The restrainer brackets support the inlet mixer by attaching the inlet mixer to the adjacent jet pump riser pipe.
The gravity actuated wedge maintains contact between the inlet mixer and the restrainer bracket. More specifically, the wedge works in cooperation with two set screws which are tack welded to the restrainer bracket to maintain contact with the inlet mixer. The flow of water through the jet pumps typically includes pressure fluctuations caused by various sources in the reactor system. The pressure fluctuations may have frequencies close to one or more natural vibration modes of the jet pump piping. The jet pump piping stability depends on the tight fit-up, or contact, of the restrainer brackets and the inlet mixers.
Operating thermal gradients, hydraulic loads, and fluctuations in the hydraulic loads can overcome the lateral support provided by the gravity wedge, allowing gaps or clearances to develop at the opposing two fixed contacts or set screws. Particularly, the tack welds can break and the set screws can loosen permitting the jet pump to vibrate within the restrainer bracket. The loss of contact between the inlet mixer and the restrainer bracket can change the jet pump natural frequency to match some excitation frequency in the system, causing vibration of the piping. The vibration exerts increased loads on the piping system which may cause cyclic fatigue cracking and wear of the piping supports. This can result in degradation from wear and fatigue at additional jet pump structural supports.
Gravity wedge supports have been used at locations where clearances have developed in restrainer bracket contacts in order to overcome this problem. The gravity wedge support employs a sliding wedge and a fixed bracket mount which engages the jet pump restrainer bracket. Proper installation of the wedge support requires disassembly of the jet pumps, which is an undesirable expense and may cause an extension of reactor maintenance downtime. Additionally, the gravity wedge supports typically include bolted attachments which could vibrate loose. Another attempted solution is to reinforce the welded attachment of the two set screws to the restrainer bracket, then reset the inlet mixer against the set screws when the jet pump is reassembled. However, this procedure also requires significant downtime and disassembling the jet pumps. Neither of these modifications provide an improvement in the effectiveness of the existing gravity wedge to provide lateral support to the inlet mixer in the restrainer bracket.
It would be desirable to provide a wedge preload apparatus that attaches to the jet pump gravity wedge to improve the effectiveness of the gravity wedge. It would also be desirable to provide a wedge preload apparatus that initially adjusted to the variable positioning of the gravity wedge and maintained a preload force while compensating for the downward displacement of the gravity wedge due to possible wear after installation.